GABAergic synapses are the major inhibitory connections in the brain and likely play an important role in all brain functions, including learning and memory. A deficit in GABAergic synapses is often implicated in devastating disorders such as autism, schizophrenia, and epilepsy. Despite their importance, many fundamental questions regarding the regulation and function of GABAergic synapses remain unanswered. Our long-term goal is to explore the cellular and molecular mechanisms by which neuronal activity is coupled to modification of GABAergic synapses during behavioral experience and to understand how disruption of this form of GABAergic synaptic plasticity leads to cognitive deficits. Towards this goal, we have recently identified the activity-regulated bHLH-PAS transcription factor Npas4 as a key regulator of GABAergic synapses. Npas4 is rapidly induced by excitatory synaptic activity and its level determines the number of GABAergic synapses formed on excitatory neurons. These findings suggest that Npas4 is the molecular link between neuronal excitation and GABAergic synapses. By investigating Npas4's role in regulating activity-dependent modulation of GABAergic synapses and regulating behavioral output of neural circuits, the research plan outlined here will yield new insights into the etiology of many neurological disorders.